There is currently known in the prior art a calorimetric method of measuring microwave power. Such method comprises the steps of permitting microwaves to be absorbed by a suitable medium, such as a measured quantity of water, measuring the temperature rise in the medium, and relating the temperature rise to the microwave power. In this respect, in the calorimetric method, the temperature rise in the absorption medium is proportional to the microwave power. Though currently known microwave ovens are typically adapted to generate microwave power levels that are nominally in the range of 400 to 1500 watts, the actual power generated may be less than the nominal or rated power due to the deterioration of the components of the microwave oven over time. As such, techniques for measuring microwave power are important for determining whether the microwave oven is operating at its rated output, which is often a matter of serious concern to large restaurant chains and to home cooks.
As indicated above, the technique of using a calorimeter to measure microwave power is known in the prior art, and is described in U.S. Pat. No. 2,648,047 (Hollingsworth) , U.S. Pat. No. 2,846,647 (MacPherson), U.S. Pat. No. 2,848,683 (Jones), and U.S. Pat. No. 2,866,950 (Smits). Though these particular references are directed to the use of a calorimeter to measure microwave power, they do not teach or suggest an instrument that would be practical for use in testing microwave ovens.
Though there is known in the prior art wattage detectors for use in relation to microwave ovens, such prior art detectors possess certain deficiencies which detract from their overall utility. One such prior art microwave oven wattage detector is the calorimeter disclosed in U.S. Pat No. 4,916,386 (Schultz). The Schultz reference describes a calorimeter comprising a container which holds a quantity of fluid. Immersed in the fluid is a liquid crystal thermometer. This thermometer is itself subject to inaccuracies due in part to its digital nature, and in part to the difficulty in determining which of its elements is the most brightly illuminated.
The calorimeter disclosed in the Schultz reference is used by operating the microwave oven for a pre-determined length of time while the calorimeter is disposed therein. As is well known in the prior art, microwave ovens include a timer into which the user sets the desired cooking time and which limits the cooking time to the duration set into the timer. When a calorimeter is used to check the operation of the microwave oven, as in the Schultz reference, the duration of oven operation is set in the above-described manner.
However, this particular method of determining the duration of time over which full microwave power is applied to the absorption medium of the calorimeter is subject to an inaccuracy. In this respect, as is acknowledged in the Schultz reference, there is typically a delay, on the order of several seconds, between the time when the START button of a microwave oven is pushed, and the time when the microwave magnetron generator of the microwave oven fires, thus producing the full microwave power. Though the Schultz reference characterizes such delays as being minor, measuring times on the order of 30 seconds are taught thereby. Clearly, as the on time of the microwave oven is decreased, this source of error becomes more significant. Indeed, as will be discussed in more detail below, in the calorimetric wattmeter constructed in accordance with the present invention, oven on times on the order of 10 seconds are contemplated, thus causing a time delay of several seconds between the nominal time interval and the actual time interval to become much more significant.
Another source of measurement error encountered in the use of prior art microwave oven wattage detectors arises from variations in the initial temperature of the absorption medium to be heated. In the calorimetric wattmeter constructed in accordance with the present invention, it is contemplated that the user will fill the same with tap water immediately prior to making a measurement of microwave power within the microwave oven. As will be recognized, the temperature of the tap water may vary considerably from one measurement to another. To solve this temperature variance problem, the Schultz reference teaches placing the calorimeter disclosed therein into a refrigerator prior to its use, then bringing the calorimeter out of the refrigerator and allowing the temperature thereof to rise to a certain specific temperature at which the measurement of the microwave power is initiated. Though this technique results in a constant starting temperature, it requires the extra step of chilling the absorption medium.
In summary, prior art calorimeters for use in measuring microwave oven wattage suffer from two primary disadvantages. Firstly, an initial chill-down or cooling process within a refrigerator is needed to assure that the calorimeter has a constant starting temperature at the initiation of each microwave power measurement. Secondly, the measurement process must be conducted for time periods on the order of thirty seconds so as to minimize the error caused by the delayed start-up of the microwave magnetron generator. The present invention addresses and overcomes the above-described deficiencies of the prior art microwave oven wattage detectors by providing an improved calorimetric wattmeter that does not require chilling in a refrigerator prior to being used to obtain a microwave power measurement, requires microwave oven on times as short as 10 seconds to obtain a microwave power measurement, and eliminates the error caused by the delayed firing of the microwave magnetron generator of the microwave oven.
Regarding one additional aspect of the calorimetric wattmeter of the present invention, it should be noted that the use of a neon bulb or lamp as an indicator of the presence of microwave radiation is known in the prior art, and is described in U.S. Pat, No. 3,748,424 (Fitzmayer), U.S. Pat. No. 4,529,855 (Fleck), and U.S. Pat. No. 2,408,198 (Coltman).